882 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



the system equalization. Obviously, however, the elimination of gain 

 adjusting elements puts a higher premium than ever on the require- 

 ment that all elements, including tube capacities, show only small 

 random variations about tightly controlled nominal values. 



In addition to the gain requirements discussed above, the following 

 line ampUfier objectives are to be met: 



1. The gain of the amplifier must be continuously variable, under the 

 control of a regulator circuit, to compensate for differences in lengths of 

 cable sections and for variations of cable loss caused by temperature 

 changes. The shape of the gain change should match the square root of 

 frequency shape of the line loss change over the transmitted band to 

 within a few hundredths of a db. This accuracy of shape, which of course 

 is based on equalization considerations, should hold over the entire 

 regulation range, which is about ±6 db at the top transmitted frequency. 



2. The input and output impedances of the amplifier should match 

 the cable impedance in order to minimize the effects, on television trans- 

 mission, of echoes caused by line irregularities such as splices. Tolerable 

 values of reflection coefficient at amplifier input and output are about 

 5 per cent at the television carrier and 10 to 15 per cent at upper band 



3. Feedback consistent with system modulation requirements, shaped 

 across the transmitted band to minimize low frequency intermodulation 

 products and to give a smooth, easily equalized shape of gain change 

 as tubes age, must be obtained while maintaining adequate margins 

 against singing. 



4. Other requirements include design and selection of elements to 

 assure as small a change of gain versus ambient temperature as possible, 

 mechanical design provisions for keeping the temperature rise within the 

 unit to a minimum both in order to keep the temperature-gain effect 

 small and to obtain long element life, a sealed housing to avoid damage by 

 humidity in exposed locations, and provision of facilities for testing tubes 

 in service. 



Configuration 



The circuit configuration of the line ampUfier is shown in Fig. 1. 

 It consists of two independent feedback amplifiers with a regulating net- 

 work between them acting like a two-terminal interstage. Each of 

 the amplifiers is essentially a two-stage circuit — the input amplifier 

 literally so and the output amplifier essentially so since the double- 

 triode output circuit acts like a single stage. Each amplifier is connected 

 to the coaxial through a coupling network which consists of a transformer 



